1. Field of the Invention
The present invention is directed to a copying machine, and more particularly, relates to an image forming apparatus which scans the image of a document, converts it into an electric signal and forms an image on a sheet of paper, in accordance with the signal.
2. Description of the Related Art
Recently, copying machines using a thermal transfer type ink ribbon have been developed and put to practical use. These machines scan a document image by means of a CCD line sensor, quantize a signal corresponding to the document image output from the line sensor, using a digital technique, and transfer ink from an ink ribbon onto a sheet of paper, in accordance with the quantized signal.
Recently developed copying machines of this type are equipped with a magnification function for altering the magnification (reduction and enlargement) from 70 to 150% of normal size in units of 1%.
However, when magnified copies are made by such copying machines, a moire effect or the like is liable to occur, resulting in an unnaturally rendered image being printed as a consequence.
In providing enlarged copies using this type of copying machine, an image on a given dot is simply read and printed plural number of times according to the set magnification. In providing reduced copies, on the other hand, several dots of a document image are thinned out according to the set ratio of reduction. Thus, according to the above-described conventional copying machines, the density of a reduced or enlarged image at one dot position is the density of the original image itself and is not changed according to the magnification set. Consequently, even if a moire effect arising from interference between the frequency characteristic of a document image and that of the line sensor has been compensated for at the time of signal quantization, it still may occur through changing the magnification.